The present invention relates to an antenna device commonly used for two frequencies in short distance communication.
Conventionally, a wireless tag reader system has been put into practical use in which a wireless ID tag (responder) is attached to a commercial product, an article, or the like, and a unique identification data that is previously set in the wireless ID tag is wirelessly read by an ID reader. Also another system such as an automatic ticket gate system has been put into practical use in which a data stored in an IC card or the like in place of such an ID tag is wirelessly read by a reader.
FIG. 3 shows an example of a general configuration of a wireless tag system. An ID reader 10 comprises a transmitter 11 and a receiver/demodulator 12. The transmitter 11 amplitude-modulates a transmission carrier wave with an inquiry signal and a clock signal, and outputs the modulated signal. The transmission signal output from the transmitter 11 is supplied to an antenna 15 via a directional coupler 13 and a circulator 14, and then transmitted from the antenna 15 to a wireless ID tag 20. For example, a loop antenna is used as the antenna 15 of the ID reader 10 (for example, see Japanese Patent Publication No. 9-98014A). A part of the output signal of the transmitter 11 is supplied to the receiver/demodulator 12 via the directional coupler 13.
Generally, the wireless ID tag 20 is configured with using an IC chip in the following manner. A radio wave transmitted from the antenna 15 of the ID reader 10 is received by a tag antenna 21. A driving power is generated from the received radio wave to operate an internal logic circuit. The unique identification data which is previously stored in a memory is read out, and the transmission carrier wave transmitted from the ID reader 10 is amplitude-modulated therewith. The modulated carrier wave is reradiated to the ID reader 10 as a return wave.
The ID reader 10 receives at the antenna 15 the return wave from the wireless ID tag 20, and supplies the received wave to the receiver/demodulator 12 via the circulator 14. The receiver/demodulator 12 extracts the clock signal from the output signal of the transmitter 11 which is supplied via the directional coupler 13, demodulates the identification data of the wireless ID tag 20, converts the demodulated data into a digital data, and sends the digital data to a host apparatus such as a personal computer (not shown).
In this way, the ID reader 10 can read the identification data of the wireless ID tag 20, and check the contents of the data.
In FIG. 3, the example in which the identification data of the wireless ID tag 20 is checked by the ID reader 10 is shown. Recently, also a system in which the single ID reader 10 can check not only the identification data of the wireless ID tag 20, but also that of another medium such as an IC card has been proposed. In this case, for example, a frequency of 2.4 GHz is used for identifying the data of the wireless ID tag 20, and that of 13.56 MHz is used for identifying the data of the IC card. Therefore, antennas respectively for 2.4 GHz and 13.56 MHz must be prepared as the antenna 15 of the ID reader 10.
In the case where the ID reader 10 identifies not only the data of the wireless ID tag 20, but also that of another medium using a different frequency as described above, it is required to use two antennas respectively corresponding to the use frequencies. This impedes reduction of the size of an apparatus. Recently, it has been attempted to provide a personal digital assistant with the function of the ID reader 10, and an antenna is requested to reduce its size.